[Polyworld-develop] graph theory metrics check-in

[Larry Y. <la...@in...>]

The source code check-in that was just done for Xin Shuai's support 
for calculating and plotting graph theoretical metrics has a few 
issues:

* SciPy, NumPy, and NetworkX are probably now required to run 
CalcMetricOverRecent

* There is sufficient commonality between CalcMetricOverRecent and 
CalcComplexityOverRecent, and between common_complexity.py and 
common_metric.py, that these pairs of scripts could probably be 
merged, but I did not attempt it for this first check-in.

* I'm not sure "value" is the best variable name to stand in for 
"complexity" and "metric", but left this as Xin had it in the plot 
script.

* I solved a few issues, such as zero-length lists, a bit differently 
than Xin did, so Xin needs to confirm that everything still works 
after my changes.

- larryy